where the dot and the prime stand for the derivative with respect
to the time and \(x=\ln {a}\) , respectively. Considering the Hubble
radius as the IR cutoff, it is easy to show that \(r\)  is a constant
[1]}, [2]}. Here, we use the future event horizon
as the IR cutoff and so \(r\)  is no longer a constant [3]}.
The future event horizon is defined as [3]}, [5]}
\(R_{h}=a(t)\int _{t}^{\infty }{\frac{dt}{a(t)}}=\frac{c\sqrt{1+r}}{H},\) 
